[llvm] [VPlan] Explicitly handle scalar pointer inductions. (PR #83068)

[Florian Hahn via llvm-commits]

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@@ -547,12 +552,42 @@ static VPValue *createScalarIVSteps(VPlan &Plan, const InductionDescriptor &ID,
 /// provide them by building scalar steps off of the canonical scalar IV and
 /// update the original IV's users. This is an optional optimization to reduce
 /// the needs of vector extracts.
+/// If all users of VPWidenPointerInductionRecipe only use its scalar values,
+/// replace it with a PtrAdd (IndStart, ScalarIVSteps (0, Step)).
 static void optimizeInductions(VPlan &Plan, ScalarEvolution &SE) {
   SmallVector<VPRecipeBase *> ToRemove;
   VPBasicBlock *HeaderVPBB = Plan.getVectorLoopRegion()->getEntryBasicBlock();
   bool HasOnlyVectorVFs = !Plan.hasVF(ElementCount::getFixed(1));
   VPBasicBlock::iterator InsertPt = HeaderVPBB->getFirstNonPhi();
   for (VPRecipeBase &Phi : HeaderVPBB->phis()) {
+    // Replace wide pointer inductions which have only their scalars used by
+    // PtrAdd(IndStart, ScalarIVSteps (0, Step)).
+    if (auto *PtrIV = dyn_cast<VPWidenPointerInductionRecipe>(&Phi)) {
+      if (!PtrIV->onlyScalarsGenerated(Plan.hasScalableVF()))
+        continue;
+
+      const InductionDescriptor &ID = PtrIV->getInductionDescriptor();
+      VPValue *StartV = Plan.getVPValueOrAddLiveIn(
+          ConstantInt::get(ID.getStep()->getType(), 0));
+      VPValue *StepV = PtrIV->getOperand(1);
+      VPRecipeBase *Steps =
+          createScalarIVSteps(Plan, InductionDescriptor::IK_IntInduction,
+                              Instruction::Add, nullptr, SE, nullptr, StartV,
+                              StepV, InsertPt)
+              ->getDefiningRecipe();
+
+      auto *Recipe =
+          new VPInstruction(VPInstruction::PtrAdd,
+                            {PtrIV->getStartValue(), Steps->getVPSingleValue()},
+                            PtrIV->getDebugLoc(), "next.gep");
+
+      Recipe->insertAfter(Steps);
+      PtrIV->replaceAllUsesWith(Recipe);
+      continue;
+    }
+
+    // Replace widened induction with scalar steps for users that only use
+    // scalars.
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fhahn wrote:

I think try to share `createScalarIVSteps` for both would make things more complicated, as they have a lot of different arguments. left as is for now.

https://github.com/llvm/llvm-project/pull/83068